The present invention relates in particular to a transmission system that includes a transmission shaft presenting two longitudinal ends, in which a connection member—such as a deformable coupling member—is fitted to each end of the shaft, the two connection members serving to connect the shaft to two respective rotary members of a rotorcraft such as a main gearbox (MGB) and a tail rotor gearbox (TRG) to enable the shaft to drive a rear—or “tail”—rotor of the rotorcraft by means of the TRG, via the MGB.
Patents FR 2 817 234 and U.S. Pat. No. 6,680,554 describe such a power transmission system between two gearboxes of a helicopter, the system comprising a (hollow) tube acting as a transmission shaft.
The shaft is said to be “supercritical” when the frequency corresponding to a speed of rotation of the shaft (in its normal operating range) is higher than the lowest resonant (natural) frequency of the shaft.
An active magnetic damper is placed between the two gearboxes; the damper is controlled by a computer making use of signals delivered by a sensor that is responsive to the lateral (radial) position of the shaft and that includes a proportional integral differential (PID) regulator for damping the vibrations of the shaft as it passes through resonant frequencies.
That magnetic damper corresponds to an active radial magnetic bearing that damps the vibrations of the shaft, that presents stiffness that is low enough to avoid shifting the natural frequencies of the shaft by more than 3%, and that presents a clearance of 3 millimeters (mm) to 4 mm relative to the shaft.
In particular because of the complex and critical nature of the hardware and software components of an active magnetic bearing, implementing such components on board a rotorcraft suffers from the large development costs needed to enable them to be qualified or certified.
Patents EP 1 068 665 et U.S. Pat. No. 6,268,676 describe a shaft-support system comprising a passive radial magnetic bearing in which the magnetic stator portion (i.e. nonrotary portion) is connected to a support by four elastomer dampers. The mass and the stability of such a system make it difficult to use for supporting a rotorcraft transmission shaft.